qt: Introduce PlatformStyle

Introduce a PlatformStyle to handle platform-specific customization of
the UI.

This replaces 'scicon', as well as #ifdefs to determine whether to place
icons on buttons.

The selected PlatformStyle defaults to the platform that the application
was compiled on, but can be overridden from the command line with
`-uiplatform=<x>`.

Also fixes the warning from #6328.

src/init.cpp

@@ -441,6 +441,9 @@ std::string HelpMessage(HelpMessageMode mode)
         strUsage += HelpMessageOpt("-min", _("Start minimized"));
         strUsage += HelpMessageOpt("-rootcertificates=<file>", _("Set SSL root certificates for payment request (default: -system-)"));
         strUsage += HelpMessageOpt("-splash", _("Show splash screen on startup (default: 1)"));
+        if (showDebug) {
+            strUsage += HelpMessageOpt("-uiplatform", "Select platform to customize UI for (one of windows, macosx, other; default: platform compiled on)");
+        }
     }
 
     return strUsage;